Thermocouple hygrometer



Filed April 16, 1934 INVENTORS. Fran/r /fe/m Edward l?. Wayne MM ATTORNEY.

Patented Aug. 30, 1938 UNITED STATES PATENT OFFICE THEBMOCOUPLE HYGBOMETEB Application April 16, 1934, Slal No. 720,698

ihiml.

This invention relates to temperature measuring devices, and, more particularly, relates to a thermocouple hygrometer.

Where accuracy is desired, relative humidity is determined by interpolation of charts or by calculations, from simultaneous measurements of temperatures of air and oi the surface of water over which the same air is rapidly passed. I'he latter temperature is known as the temperature lo of evaporation, or the wet-air temperature. 'Ihe standard instrument oi reference for these measurements is the sling psychrometer, an apparatus in which two thermometers in a sling are caused to be whirled rapidly in the air for which a del termination of relative humidity is desired. One of the two thermometers is oi the ordinary type with thin glass bulb while the other is similar but has its bulb encased in a moistened wick, and its readings are known as the wet bulb or wetair temperatures. The simultaneous readings of the wet-air thermometer, and of the other, the "dry bulb or dry-air thermometer, permit a determination ot relative humidity by reference to standard hygrometer tables, charts, or formulae. It is easily seen that the process of manually whinling the sling psychrometer is slow and laborious, is subject to error and risk of breakage of thermometers, and the device itself is not adaptable to automatic recording of readings. Heretofore, to overcome these defects, Vresistance thermometers have been utilized, one oi two being provided with a wick and both being placed in a stream of air whose relative humidity is desired. These resistance thermometers have been connected to suitable temperature recorders or other recording devices, from whose records the relative humidities of the air could be determined for any desired time. However, resistance thermometers are relatively bulky, the 4o wicks necessary for the wet-air thermometer being correspondingly large, and the volumel of water necessary to keep theV wicks constantly moist being so great as to require a continuously running or often replenished supply. Hygrometers of thistype are large, heavy, clumsy, and require frequent servicing. These devices, also, are inaccurate, are dimcult to keep in satisfactory operating condition, and require a speciallyguided stream of air over the wet-air thermo- '30 meter.

The primary object oi our invention is to provide a recording hygrometer in which these defects are overcome.

Another object of our invention is to provide 55 a wet-air thermocouple.

(Cl. 'i3-338) Still another object of our invention is to provide a wet-air thermocouple in which the measuring junction is maintained substantially at the wet-air temperature.

A further object o our invention is to produce 5 an automatic hygro meter which is accurate, occupies a. minimum oi space, requires very little water, and needs practically no attention or eervicing.

Btill a further object of our invention is to m provide an improved device oi' the character described, of simple, cheap, and eiiicient construction, having a maximum eiilciency.

With these and other objects in view, which will become apparent as the description proceeds, 15 our invention is embodied in a temperature recording system consisting essentially oi two thermocouples located in a stream of air whose relative humidity is desired to be known and suitably connected to a potential recording de- 20 vice, one thermocouple being of the ordinary type and the other being a. wet-air thermocouple comprising a junction of two metallic conductors having diiierent thermoelectric powers, em- 'bedded within a heat-controlling metallic mem- 25 ber, said metallic member being closely enclosed within a wet wick and the heat conductance o! said conductors being very small compared with that of said member.

The invention, both as to details of construc- 30 tion and combination of parts, will best be understood from the following description of a specitlc embodiment which we illustrate as an example, when read in connection with the accompanying drawing, in which: u

Fig. 1 is a sectional elevational view oi our wet-air thermocouple.

Fig. 2 is a fragmentary elevational view, partly in section, oi' a wet-air and dry-air thermocouple assembly with fan and moisture resero voirs.

Referring now to the drawing. we have shown in Fig. 1 an embodiment of our invention which illustrates the principle of our wet-air thermocouple. Consider a wet-air thermocouple constructed according to Fig. l, in which a metal wire or conductor i2 is united with another conductor Ii of diti'erent metal to form a temperature-sensitive measuring junction IB, the conductors I2 and Il being twisted together for a l0 few turns beyond said `iunction. This twisted portion lies within a cylindrical bore il of a metal cylinder, block, or member 2li, and the junction I6 is placed in a thermally-conducting heat-interchanging relationship with said mem- 56 ber 20 by lling the space between the twisted portion of said conductors and member 20 with solder or other solidified metal 22. An electrically-insulating cement or other electrically-insulating water-impervious material 24 surrounds the conductors I2 and i4 outwardly from the Junction Il for a short distance. A cloth wick 2B is wrapped about the member 2li, the Junction i6. and part of the cement-covered portions of conductors i2 and I4, and is secured by twisted loops of wire 20 on either side of the member 20.

Let the wick b'e saturated with water and a stream of air be forced over the assembly. The temperature of the outer wet surface of the wick thereupon is reduced by evaporation of the water to the temperature of evaporation, or wet-air temperature, T'. Heat thereupon ows by conduction from the member 2l through the water permeating wick 2B and the interstices between said member and wick. The temperature of the surface of member 2li is thus reduced to some new temperature Tm, higher than '1*'. Heat will now flow into the member 20 through the conductors i2 and i4. whose outer extremities are at the dry-air temperature, Td, with the result that the Junction I6 will be at a temperature T1,

intermediate of temperatures Td and Tm. Equilibrium will be established with a conduction flow of a constant quantity of heat Q from the outer extremities of conductors i2 and I4 to the wet surface of the wick, where it will be removed by evaporation of water into the air stream.

Let the heat conductances of the heat paths be represented as follows:

K1z=heat conductance of conductor l2 K14=heat conductance of conductor I4 Km=heat conductance of member 20 from Junction i8 to wick 20 Kw=heat conductance from outer surface of member 20 to outer surface of the water-saturated wick.

Let time be represented by the symbol t. Then the rate of heat flow from the conductors through the thermocouple to the air stream, is:

Solving the above equations simultaneously for T, we obtain The-E. M. F. at the cold or outer junction of the conductors I2 and i4 (not illustrated in the drawing) is a well-known function of the temperature of the junction, T1. To the extent that the temperature T1, measured by the thermocouple, is greater than the wet-air temperature Tw, the wet-air temperature reading of the thermocouple will be in error.

From Equation (4), above, it is at once seen that T; cannot equal Tw, for in that case K12 plus K14 must equal zero, or Kw and Km must each be innite, either of which conditions is a physical impossibility. The values for K12 and K14 can be made quite small, of course, but the minimum is limited by the electric conductance required to obtain the sensitivity of measurement desired. The constants Kw and Km cannot, of course, be made to equal infinity, but their magnitudes may be increased practically to an extent such that the right-hand term of Equation (4) reduces to T' within any desired practical degree of error.

E' may be increased by reducing the thickness of the wick and by making the wick nt snugly over a metallic surface. The value of Kw varies directly as the area of such surface. and thus can be increased indennitely by increase in such area. Km may be suitably increased by choosing a metal of high heat conductivity and by using a large cross-section of heat-dow path to junction Il.

It is clear from the foregoing that by choosing any desired values of Kw and Km, the flow ot heat from the thermocouple Junction to the wick can be suitably controlled.

Referring, now to Fig. 2,. the wet-air thermocouple of Fig. l is shown with the end I2 oi' wick 28 extended into a water reservoir Il. The reservoir 24 consists of a short wide-mouthed bottle and is provided with a stopper 3B through which extends a tube Il providing ingress for the wick 2B. A tube lil extending to a point adjacent the bottom of the reservoir 24 and through the stopper 20 is connected by a flexible tube 42 to a tube 44 inserted through a stopper 46 in an opening 48 at the bottom of a main water reservoir B0 mounted vertically above reservoir 34 on a stand 52, by a clamp B4. A tube iii similarly connects the upper space of the reservoir 34 and the top of the main reservoir 50 by way of a exible tube 5B, through a top opening G0 in reservoir 5G and a stopper 2'therein inserted.

A fan B4 is mounted in such manner that the stream of air directed by the outer portion of its blades impinges onthe wet-air thermocouple. A dry-air thermocouple 66, oi.' the usual construction, is 'mounted at the same distance from the fan blades as the .wet-air thermocouple, but in such position that it receives the air impeiled by the blade opposite that which forces air over the wet-air thermocouple. In each case, care is taken to insure that the air heated by passage through the fan motor does not impinge on the thermocouples. Conductors B8 and 'lil of thermocouple BB. and conductors I2 and i4 are led either to appropriate indicating or recording voltmeters, or to indicating or recording potentiometers (not illustrated) The operation of the device is as follows: Water is placed into the reservoir 50', the tube 42 being pinched meanwhile so that reservoir 34 will not be flooded. The stopper 62 is then replaced in the top opening 60 and the tube 42 is released, whereupon the reservoir 34 fills to a point slightly above the opening leading into tube 56 in said reservoir 34 and is continuously maintained at that level until reservoir 5B is emptied. The fan B4 is started and thereupon projects a stream of air over the thermocouples, the dry-air thermocouple registering the actual temperature of the air stream. In the case of the wet-air thermocouple the stream of air from the fan evaporates water from the wet wick 28 and in doing so extracts heat therefrom, thus reducing the temperature of the wet surface of the wick to Tw. Due to the difference in temperature Tw of the water surface and the initial temperature Ts of the entire system, heat flows from the conductors I2 and il, through the member 20. and through the moisture-saturated wick 28 to the wet surface from which the heat is being extracted by the air stream. After steady state has been reached, i. e., after the excess heat stored in the system has been removed, equilibrium temperatures will be reached, in which, in our wet-air thermocouple, the temperature T3, which determines the E. M. F. generated, is substantially the same as Tw, as heretofore explained.

A wet-air thermocouple constructed according to Fig. 1, in which the member 2li was of copper and of mass approximately 2 grams, and the thermocouple conductors I2 and Il were of copper and oi' constantan respectively oi No. 28 A. W. G., gave satisfactorily accurate determinations of relative humidity as compared with simultaneous averaged readings with a sling psychrometer. As the purpose of the small gage conductors is to prevent heat conduction to the wet-air thermocouple, these conductors need not be limited to such small size for any great length, and at a distance oi a few inches from the thermocouple it is recommended that they be connected to conductors of larger gage to convey the electrical potential to the recording or indicating device, so that electrical sensitivity of the system be not impaired.

Although the member 20 has been illustrated in Fig. 1 as of cylindrical shape, it may instead be of any desired shape, a form which permits better and more uniform contact with the wick, such as a spheroid, being even more desirable than that illustrated. Furthermore, as the heat conductance Kw is probably less susceptible of being easily increased to a particular desired magnitude than the heat conductance Km, it is quite feasible to obtain desired large values of these quantities without disproportionately increasing the mass of the member, by making said member hollow and providing it with heat conducting struts or ribs converging to the thermocouple junction located within said member.

Although the dry-air thermocouple 88 is shown as symmetrical to the wet-air thermocouple with respect to the fan-propelled air stream, the system will operate with equal effectiveness if the thermocouple 86 be placed anywhere in the said air stream except in that portion heated by the fan motor or very close or to leeward of the wetair thermocouple. There is no error introduced by placing the dry-air thermocouple BB in a direct line with and between the fan and the wet-air thermocouple.

The hygrometer system of Fla'. 2 is merely representative, and may be variously assembled, for example. the wet-air and dry-air thermocouples may be placed in a duct or iiue of moving air instead of before a Ian.

Although the invention has been described in considerable detail. such description is intended as illustrative rather than limiting, as numerous embodiments will be apparent to those skilled in the art. Our invention. therefore, is not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

We claim:

1. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions, a thermocouple unit enclosed within said envelope, said thermocouple unit including a heat conductive body enveloping the thermocouple junction, said body being proportioned to the evaporative properties of the envelope such that the heat conducted into the interior of said envelope by the elements of said thermocouple is removed rapidly enough to maintain the temperature of the thermocouple junction substantially the same as that of the surface oi said envelope.

2. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions, a thermocouple unit enclosed within said envelope, said thermocouple unit including a heat conductive body arranged in thermally conductive relation to the thermocouple junction. said body.being proportioned to the evaporative properties oi the envelope such that the thermocouple junction is cooled substantially to the true wet-air temperature of the surface of the envelope.

3. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions, a thermocouple unit enclosed within said envelope, said thermocouple unit including a thermocouple junction, a heat dissipating element and a pair of leads. said leads constituting the sole means by which heat is introduced into the interior of said envelope, and said element being proportioned to the evaporative properties of the envelope such that the heat conducted into the interior or said envelope by said leads is removed rapidly enough to maintain the temperature of the thermocouple Junction substantially the same as that of the surface oi said envelope.

4. In a hygrometer, a wet air-unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions and a thermocouple unit enclosed within said envelope. the leads oi said thermocouple unit constituting the sole means by which heat is introduced into the interior oi said envelope. and the thermocouple unit and the thermal conductance of said leads being proportioned to the evaporative properties of the envelope such that the heat conducted into the interior of said envelope by sali leads is removed rapidly enough to maintain the temperature of the thermocouple Junction substantially the same as that of the surface oi' said envelope FRANK KAHN. @WARD R. WAYNE.

CERTIFICATE 0F CORRECTION Patent No. 2,128,1462.

August 50, 1958 FRANK KAHN, El AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3,\second column, line 50, claim li, for the word "sail" read @alimenti that the said Letters Patentshouldbe read with this correction therein that the saine may conform to the record of' the case in the Patent Office.

Signed and sealed this 25th day of October, A. D. 195B.

(Qon1\ Henry .Van ,Arsdale Acting Commissioner of Patents.

to Fig. 1, in which the member 2li was of copper and of mass approximately 2 grams, and the thermocouple conductors I2 and Il were of copper and oi' constantan respectively oi No. 28 A. W. G., gave satisfactorily accurate determinations of relative humidity as compared with simultaneous averaged readings with a sling psychrometer. As the purpose of the small gage conductors is to prevent heat conduction to the wet-air thermocouple, these conductors need not be limited to such small size for any great length, and at a distance oi a few inches from the thermocouple it is recommended that they be connected to conductors of larger gage to convey the electrical potential to the recording or indicating device, so that electrical sensitivity of the system be not impaired.

Although the member 20 has been illustrated in Fig. 1 as of cylindrical shape, it may instead be of any desired shape, a form which permits better and more uniform contact with the wick, such as a spheroid, being even more desirable than that illustrated. Furthermore, as the heat conductance Kw is probably less susceptible of being easily increased to a particular desired magnitude than the heat conductance Km, it is quite feasible to obtain desired large values of these quantities without disproportionately increasing the mass of the member, by making said member hollow and providing it with heat conducting struts or ribs converging to the thermocouple junction located within said member.

Although the dry-air thermocouple 88 is shown as symmetrical to the wet-air thermocouple with respect to the fan-propelled air stream, the system will operate with equal effectiveness if the thermocouple 86 be placed anywhere in the said air stream except in that portion heated by the fan motor or very close or to leeward of the wetair thermocouple. There is no error introduced by placing the dry-air thermocouple BB in a direct line with and between the fan and the wet-air thermocouple.

The hygrometer system of Fla'. 2 is merely representative, and may be variously assembled, for example. the wet-air and dry-air thermocouples may be placed in a duct or iiue of moving air instead of before a Ian.

Although the invention has been described in considerable detail. such description is intended as illustrative rather than limiting, as numerous embodiments will be apparent to those skilled in the art. Our invention. therefore, is not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

We claim:

1. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions, a thermocouple unit enclosed within said envelope, said thermocouple unit including a heat conductive body enveloping the thermocouple junction, said body being proportioned to the evaporative properties of the envelope such that the heat conducted into the interior of said envelope by the elements of said thermocouple is removed rapidly enough to maintain the temperature of the thermocouple junction substantially the same as that of the surface oi said envelope.

2. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions, a thermocouple unit enclosed within said envelope, said thermocouple unit including a heat conductive body arranged in thermally conductive relation to the thermocouple junction. said body.being proportioned to the evaporative properties oi the envelope such that the thermocouple junction is cooled substantially to the true wet-air temperature of the surface of the envelope.

3. In a hygrometer, a wet-air unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions, a thermocouple unit enclosed within said envelope, said thermocouple unit including a thermocouple junction, a heat dissipating element and a pair of leads. said leads constituting the sole means by which heat is introduced into the interior of said envelope, and said element being proportioned to the evaporative properties of the envelope such that the heat conducted into the interior or said envelope by said leads is removed rapidly enough to maintain the temperature of the thermocouple Junction substantially the same as that of the surface oi said envelope.

4. In a hygrometer, a wet air-unit comprising an evaporative envelope adapted to be subjected to standard evaporating conditions and a thermocouple unit enclosed within said envelope. the leads oi said thermocouple unit constituting the sole means by which heat is introduced into the interior oi said envelope. and the thermocouple unit and the thermal conductance of said leads being proportioned to the evaporative properties of the envelope such that the heat conducted into the interior of said envelope by sali leads is removed rapidly enough to maintain the temperature of the thermocouple Junction substantially the same as that of the surface oi' said envelope FRANK KAHN. @WARD R. WAYNE.

CERTIFICATE 0F CORRECTION Patent No. 2,128,1462.

August 50, 1958 FRANK KAHN, El AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 3,\second column, line 50, claim li, for the word "sail" read @alimenti that the said Letters Patentshouldbe read with this correction therein that the saine may conform to the record of' the case in the Patent Office.

Signed and sealed this 25th day of October, A. D. 195B.

(Qon1\ Henry .Van ,Arsdale Acting Commissioner of Patents. 

